A capacitive touch sensor is applicable to any existing product using a traditional mechanical switch, especially to a small portable product, such as menu control buttons of a fresh model of mobile phone or media player. Reliable and cost effective capacitive touch sensors can be used to change the style of these advanced menu control switches easily so as to offer users a fully new feeling in operation. Application of capacitive touch sensors in a white appliance can prevent water and oil from eroding press keys and the appliance per se so as to improve user-friendliness and safety of the appliance. A further advantage of the replacement of traditional mechanical switches by capacitive touch sensors is more simplified manufacturing and assembling processes. The traditional mechanical switches each have to be inserted manually into a special hole in a plastic casing, while a single capacitive touch sensor board incorporating all these switches can be disposed in place below the plastic casing in one step. A mounting hole for the sensor board, which incorporates a locating slot, and some glue are sufficient to mount and align the sensor board in place. As mixed signal technologies advance, capacitive touch sensors have become a practical and value-added alternative to mechanical switches in various electronic products.
A simple parallel-plate capacitor is provided with two plates as illustrated in FIG. 12a. The two plates are separated by a dielectric layer, and thus there is an inherent sensor capacitance CSENSOR between the upper and lower plates. The sensor capacitance here refers to an inherent capacitance of the touch plate relative to the ground before the plate is touched. When a human finger touches the plate as illustrated in FIG. 12b, the human body is equivalent to the ground and the area of the plate relative to the ground is increased, resulting in a capacitance increase ΔC denoted with the broken line in the figure.
The operation principle of a capacitive touch sensor lies in sensing and converting this capacitance increase ΔC into data to be processed by a processor. A charge transfer method, a relaxation oscillation counting method and a sigma-delta Analogue-to-Digital Conversion (ADC) method are used for existing capacitive touch sensors and have their own advantages, but they generally have the drawbacks of a complex structure, large power consumption, time-consuming processing, a large number of peripheral elements and a limited ability in interference recognition. Touch sensors charged with a constant current source or constant voltage source are widely applied at present.
A capacitive touch sense control circuit based upon a constant current source charging principle as illustrated in FIG. 1 includes a current setting resistor 101, a pad capacitor 102, a start comparator 103, an end comparator 104, a Time-to-Digital Converter (TDC) 105, a voltage buffer 106 and a current mirror 107. In particular, CS is a total capacitance at the pad including a parasitic capacitance and a sensor capacitance at the pad of a chip. A reference voltage VREF results in a reference current I1 through the voltage buffer 106 and the current setting resistor 101. The voltage buffer 106 includes an error amplifier and a source followed NMOS transistor. I1 is converted by the current mirror 107 into a charging current I2 to charge the capacitor. When the capacitor pad reaches a nominal level VSTA, the start comparator 103 is inverted and the Time-to-Digital Converter (TDC) 105 starts to count. When the capacitor pad reaches a higher nominal level VEND, the end comparator 104 is inverted and the TDC stops counting. This interval of time is converted into data to be delivered to and processed by a processor.
From Q=I*t=U*C, the following formulae can be derived:T1=(VEND−VSTA)CS/I2 I2=mI1=mVREF/RS T1=RSCS*(VEND−VSTA)/mVREF 
where m is a mirror coefficient of the current mirror, and the voltages VEND, VSTA and VREF are generated by dividing the same reference voltage, so (VEND−VSTA)/mVREF is a constant, denoted with β.T1=βRSCS T2=βRS(ΔC+CS)ΔT=T2−T1=βRSΔC 
FIG. 2 illustrates this charging process depicting a charging curve L1 before the sensor capacitance varies and a charging curve L2 after the sensor capacitance varies.
A capacitive touch sense control circuit based upon a constant voltage source charging principle as illustrated in FIG. 3 includes a current setting resistor 101, a pad capacitor 102, a start comparator 103, an end comparator 104 and a Time-to-Digital Converter (TDC) 105. In particular, CS is a total capacitance at the pad including a parasitic capacitance and a sensor capacitance at the pad of a chip. A charging voltage VDD results in a reference current I1 through the current setting resistor 101 to charge the capacitor. When the capacitor pad reaches a nominal level VSTA, the start comparator 103 is inverted and a Time-to-Digital Converter (TDC) 105 starts to count. When the capacitor pad reaches a higher nominal level VEND, the end comparator 104 is inverted and the TDC stops counting. This interval of time is converted into data to be delivered to and processed by a processor.
From Ut/U0=e−t/(RC), the following formulae can be derived:T1=RSCS(ln VEND−ln VSTA)The voltages VEND and VSTA are generated by dividing the same reference voltage, so (ln VEND−ln VSTA) is a constant, denoted with α.T1=αRSCS T2=αRS(ΔC+CS)ΔT=T2−T1=αRSΔC 
FIG. 4 illustrates this charging process depicting a charging curve L1 before the sensor capacitance varies and a charging curve L2 after the sensor capacitance varies.
External electromagnetic variations may cause very often a capacitance variation of a capacitor, and an abnormal variation of the capacitor may influence the normal operation of the capacitive touch sense control circuit. Since external interferences vary differently, the constant current source charging mode is sometimes more effective for interference recognition than the constant voltage source charging mode while the latter is more effective than the former at other times. The problem to be addressed by the invention is how to provide both the constant voltage source charging mode and the constant voltage source charging mode for the purpose of more effective interference recognition.
Further, in practical applications, a hand touch may give rise to a capacitance variation of approximately 1 pF, i.e., ΔC=1 pF. The parasitic capacitance at the pad of a chip is approximately 10 pF, the external sensor capacitance is approximately 5 pF, and the reading of ΔT is only 1/16 of that of T2. In some cases, the pad parasitic capacitance and sensor capacitance may be larger while the capacitance variation resulting from a hand touch may be smaller. A more advanced application such as slide sensing or absolute position determination requires a relatively large reading obtained from this tiny capacitance variation, which requires a high precision in sensing the capacitance variation. Unfortunately, the setting resistor can not be set too large because too small a current is vulnerable to any interference, and the reading of the TDC also is limited in terms of the number of digits. Consequently, it is difficult to realize a high precision in sensing a capacitance variation in the existing capacitive touch sense control circuit structures with a large value of the inherent capacitance.